Intersection theory: correct bug, add more examples #17199

Job	Run time
doctest (1.10, ubuntu-latest)	27m 6s
test (1.6, short, ubuntu-latest)	1h 33m 51s
doctest (1.11-nightly, ubuntu-latest)	27m 19s
test (1.6, long, ubuntu-latest)	55m 2s
doctest (nightly, ubuntu-latest)	31m 45s
test (1.10, short, ubuntu-latest)	1h 12m 49s
doctest (1.10, ubuntu-latest, depwarn=error)	24m 29s
test (1.10, long, ubuntu-latest)	46m 36s
doctest (1.10, macOS, RPTU)	12m 48s
test (1.11-nightly, short, ubuntu-latest)	1h 18m 51s
test (1.11-nightly, long, ubuntu-latest)	46m 47s
test (nightly, short, ubuntu-latest)	1h 21m 14s
test (nightly, long, ubuntu-latest)	49m 1s
test (1.10, short, ubuntu-latest, depwarn=error)	1h 6m 39s
test (1.10, long, ubuntu-latest, depwarn=error)	45m 4s
test (1.10, short, macOS, RPTU)	14m 6s
test (1.10, long, macOS, RPTU)	14m 34s
test (1.10, book, ubuntu-latest)	50m 11s
test (nightly, book, ubuntu-latest)	54m 54s
	14h 53m 6s

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